1. Field of the Invention
The present invention relates to an image heating apparatus for heating an image borne by a recording material, and more particularly to an image heating apparatus adapted for use as a heat fixing device in an image forming apparatus such as a copying machine or a printer.
2. Related Background Art
In an image forming apparatus based for example on an electrophotographic process or an electrostatic recording process, a fixing apparatus as a heating apparatus for heat fixation of an unfixed toner image formed on a recording material (a transfer sheet, a printing sheet, a photosensitive sheet, an electrostatic recording sheet etc.) by a transfer process or a direct process is known in various types and configurations such as a heat roller type, a film heating type etc.
In a color image forming apparatus for forming a toner image of plural colors on a recording material, or in an image forming apparatus having a color image forming function for forming a toner image of plural colors on a recording material, a toner amount of an unfixed toner image formed on the recording material is several times larger in a full color mode than in a monochromatic mode.
As a fixing apparatus capable of satisfactory heat fixation even of an unfixed toner image of such large toner amount, there is known an apparatus employing an elastic surface roller, which is provided with an elastic layer on a fixing roller (heating rotary member) constituting a fixing member for heat fixing the toner image on the recording material.
By constructing the fixing roller as an elastic surface roller, the surface of the fixing roller undergoes elastic deformation, relative to the unfixed toner image on the recording material, corresponding to irregularities of such toner image and achieving a wrapping contact with the surface of the toner image, whereby an unfixed toner image having a large toner amount can also be heat fixed in satisfactory manner.
However, in a fixing roller equipped with an elastic layer, the elastic layer itself constitutes a heat insulating layer and there is encountered a drawback that the heat transfer efficiency deteriorates in cases where the heat source is provided inside the fixing roller.
Therefore, the present applicant already proposed, as disclosed in Japanese Patent Application Laid-Open No. 2002-236426, a fixing apparatus adopting an external heating configuration in which a heating rotary member serving as a fixing member is heated from the exterior of the fixing member, whereby the fixing member has a high heating efficiency even in case it is provided with an elastic surface, thereby ensuring a quick starting property and a fixing ability of the apparatus.
FIG. 6 shows an embodiment of a fixing apparatus with such external heating configuration for the fixing member. A fixing roller 201 serves as a fixing member maintained in contact with an image bearing surface of a recording material P, thereby heat fixing an unfixed toner image T thereon. The fixing roller 201 is an elastic surface roller of a three-layered structure, having, in succession from the inside to the outside, a metal core 211, an elastic layer 212 and a releasing layer 213.
A pressurizing apparatus 202 is in a mutual pressed contact with the fixing roller 201, thereby forming a fixing nip portion N4 for nipping and conveying the recording material P. The pressurizing apparatus 202 also serves as a heating apparatus for heating the fixing roller 201, and is provided with a rotatable cylindrical film 221, and a film/heater holder 224 for supporting the film from the inside and maintaining it in contact across a plate-shaped heater 222 thereby forming the fixing nip portion N4.
In addition to the aforementioned pressurizing/heating apparatus 202, there is provided an external heating apparatus 203 for heating the surface of the fixing roller 201 from the exterior. The external heating apparatus 203 is provided with a plate-shaped heater 232 and a heater holder 234 for supporting the heater so as to be in contact with the surface of the fixing roller 201 thereby forming a fixing roller heating nip portion N3.
The fixing roller 201 is rotated clockwise, as indicated by an arrow, by an unillustrated drive mechanism. By such rotating motion of the fixing roller 201, the cylindrical film 221 of the pressurizing/heating apparatus 202 is driven counterclockwise, as indicated by an arrow, around the holder 224, with an internal surface of the film in sliding contact with the surface of the plate-shaped heater 222 at the fixing nip portion N4.
The plate-shaped heater 222 in the pressurizing/heating apparatus 202 and the plate-shaped heater 232 in the external heating apparatus 203 are so-called ceramic heaters, of which temperature is rapidly elevated by current supplies from power supply circuits 205, 206 to heat-generating resistor layers (not shown). Temperatures of the heaters 222, 232 are detected by thermistors 223, 233 constituting temperature detecting means provided on rear surfaces of the heaters, and temperature information thus detected are supplied to a control circuit (CPU) 204.
The control circuit 204 executes a temperature control of the heater 222 by controlling the current supply from the power supply circuit 205 to the plate-shaped heater 222 in the pressurizing/heating apparatus 202 in such a manner that a temperature detected by the thermistor 223 is maintained at a predetermined heater temperature. It also executes a temperature control of the heater 232 by controlling the current supply from the power supply circuit 206 to the plate-shaped heater 232 in the external heating apparatus 203 in such a manner that a temperature detected by the thermistor 233 is maintained at a predetermined heater temperature.
In this manner the surface temperature of the fixing roller 201 is maintained at a predetermined surface temperature (fixing temperature) necessary for the heat fixation of the toner image.
As the fixing roller 201 is rotated to drive the film 221 of the pressurizing/heating apparatus 202 in a rotating motion, and as the plate-shaped heater 222 of the pressurizing/heating apparatus 202 and the plate-shaped heater 232 of the external heating apparatus 203 are powered to control the heaters 222, 232 at the predetermined heater temperatures, the surface of the fixing 201 is heated by the heat of the plate-shaped heater 222 of the pressurizing/heating apparatus 202 across the film 221 at the fixing nip portion N4, and is also heated by the heat of the plate-shaped heater 232 of the external heating apparatus 203 at the heating nip portion N3, whereby the fixing roller 201 is heated to a predetermined surface temperature (fixing temperature) required for heat fixing the toner image.
In such state, a recording material P bearing an unfixed toner image T is introduced into the fixing nip portion N4, with a toner image side at the side of the fixing roller 201, whereby the recording material P is nipped and conveyed in the fixing nip portion N4. In such conveying process, the unfixed toner image T is fixed, under a heat and a pressure, as a permanent image on the surface of the recording material P by the heat of the fixing roller 201. The recording material P emerging from the fixing nip portion N4 is separated from the surface of the fixing roller 201 and is conveyed for discharge.
Such apparatus, being provided with heating means for externally heating the surface of a fixing member such as a fixing roller, can rapidly elevate the temperature by rapid heating of only the surface, required for fixing, of the fixing member, and can improve the quick starting property and the thermal efficiency of the fixing apparatus even in case the fixing member is an elastic surface roller having an elastic layer.
In a fixing apparatus of an external heating configuration as explained in the foregoing, in order to maintain the surface temperature of the fixing roller heated by the external heating means as close as possible to a predetermined surface temperature necessary for heat fixation of the toner image thereby ensuring a fixing property for a first print immediately after the start of power supply or for all prints during a continuous sheet passing operation, it is desirable (1) to increase an electric power supplied to a heater of the external heating means, and (2) to increase a width of the heater of the external heating means.
However, an increase in the electric power supplied to the heater of the external heating means may result in a deterioration of components such as the fixing roller or the heater, or a runaway state of the heater, by a rapid temperature increase in the heater. Also a larger width of the heater of the external heating means may lead to a drawback of an increased cost of the heater.